GH-deficient (GHD) patients have increased risk of cardiovascular death and may have cardiac structural abnormalities. In non-GHD patients these are associated with cardiac autonomic dysfunction, and it is possible that autonomic dysfunction is also present in GHD patients. Power spectral analysis (PSA) of heart rate variability (HRV) indirectly measures cardiac autonomic tone and generates peaks at 3 frequency bands, very low frequency (VLF), low frequency (LF) and high frequency (HF). The area under the LF curve is considered to reflect predominantly cardiac sympathetic activity, whereas HF indicates parasympathetic activity. PSA of HRV was performed in 14 normotensive GHD patients (5 men and 9 women; mean age, 35.2 yr) and 19 healthy controls (9 men and 10 women; mean age, 38.3 yr). GHD patients had 26% lower normalized LF power (P < 0.004), 39% higher normalized HF power (P < 0.001), 28% lower normalized VLF power (P < 0.046), and 51% lower LF/HF ratio (an index of sympathovagal balance; P < 0.001) compared to controls. These data indicate that heart rate variability is abnormal in patients with GHD. The decreased sympathetic tone could be a consequence of reduced central sympathetic tone or altered cardiac responsiveness to autonomic control and may contribute to the increased cardiovascular risk in GHD patients.